Velocity interferometer system for any reflectors (VISARs), are becoming increasingly popular in the measurement of shock waves in solids and liquids. VISAR techniques are used in measurements of transit time, speed of shock waves in flight in transparent media L. C. Chhabildas and J. L. Wise, in Proceedings of the 4th APS Topical Conference on Shock Waves in Condensed Matter, Spokane, Washington, 1985, edited by Y. M. Gupta (Plenum, New York, 1986); P. M. Celliers et al., Appl. Phys. Lett. 73, 1320 (1998), and in measurements of particle velocity. However, in cases where shock compression or release may change the index of refraction n+ik of the material being studied, the VISAR technique must be applied with care. Changes in n and k introduce phase shifts into the VISAR results that are not associated with changes in velocity. This paper presents a derivation of the theoretical output of a line VISAR that includes the effects of changing n and k and an experimental observation of a non-Doppler shift related effect.